JP2003187817A - Separator for fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a fuel cell which is superior in corrosion resistance and electrical conductivity and which can manufacture at low cost. <P>SOLUTION: The separator for the fuel cell forms a first layer film made of an alloy which consists of a metallic raw material, has a contact surface with an electrode or a collector and a gas vent groove and includes Ag, Ti, Ni or Cr to their alloy on the contact surface and, furthermore, a second layer film consisting of an alloy which includes Au, Pt or Hg or their alloy on the first layer film. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a fuel cell separator which is excellent in corrosion resistance and electrical conductivity and can be manufactured at low cost.

[0002]

2. Description of the Related Art A fuel cell has a high energy conversion efficiency from fuel to electricity and does not emit harmful substances. In particular, polymer ion exchange membrane fuel cells that operate in the temperature range of 150 ° C. or lower are being actively researched, and are expected to be put into practical use in a few years. This fuel cell is capable of operating at a relatively low temperature, has a high power generation output density, and can be miniaturized. Therefore, it is suitable as a fuel cell for home use or on-vehicle use.

A polymer ion exchange membrane fuel cell usually has a fuel electrode and an oxygen electrode (air electrode) on both sides of a solid electrolyte membrane.
Are fixed to form a single cell (cell), and the cells are stacked via a plate-shaped separator provided with a ventilation groove for supplying fuel gas and air. Generally, a fluororesin ion exchange membrane having a sulfonic acid group is used as the solid electrolyte membrane, and the electrode is carbon black and water repellent.
It is formed of a dispersion of PTFE and a noble metal particle catalyst. When the hydrogen-oxygen fuel cell operates, the protons generated by the oxidation of hydrogen gas enter the electrolyte, combine with water molecules to become H ₃ O ⁺ , and move to the positive electrode side. On the positive electrode side, oxygen introduced from the ventilation groove is generated by an oxidation reaction of hydrogen and obtains an electron flowing from an external circuit, and is combined with a proton in the electrolyte to become water. By continuing these reaction processes, electric energy can be continuously taken out. The theoretical electromotive force of this unit cell is 1.2V, but in reality, electrode polarization, reaction gas crossover (a phenomenon in which fuel gas permeates the electrolyte and reaches the air electrode), and ohmic electrodes and current collectors The output voltage is about 0.6 to 0.8V due to the voltage drop due to resistance. Therefore, in order to obtain a practical output, it is necessary to stack several tens of unit cells through the separator and connect them in series.

As can be seen from the above-mentioned power generation principle, since a large amount of H ⁺ is present in the electrolyte, the inside of the electrolyte and the vicinity of the electrode are strongly acidic. Also, oxygen causes a reduction reaction on the positive electrode side.
It combines with H ⁺ to produce water, but hydrogen peroxide may be produced depending on the operating state of the battery. Since the separator is exposed to such an environment, it is required to have high chemical / electrochemical stability (corrosion resistance) in addition to electrical conductivity and airtightness.

Most of the conventional fuel cell separators are machined graphite materials. Graphite materials have low electrical resistance and high corrosion resistance, but have low mechanical strength and high processing costs. Since the separator used in the vehicle fuel cell is required to have high mechanical strength, it is difficult to apply the conventional graphite separator as it is to the vehicle fuel cell. In recent years, a method of manufacturing a separator by mixing graphite powder with a resin, injection molding, and further firing at high temperature has been studied, but airtightness is poor because the obtained fired body has a low density. Although it is possible to increase the density by immersing this separator in resin and re-baking it, the manufacturing process becomes complicated. In addition, the contact electric resistance of the separator manufactured in this way is several times higher than that of the conventional graphite separator, and the decrease in the output voltage of the battery cannot be avoided.

[0006] In addition to the graphite separator, separators made of a metal base material have been investigated. The metal separator has low electric resistance, high airtightness and mechanical strength, and is very advantageous as compared with the graphite separator. Further, when the metal base material is used, the thickness of the separator can be reduced, and thus the weight can be easily reduced. Further, the fuel cell can be further reduced in weight by using a low specific gravity metal material such as aluminum. However, the metal separator has a problem that the base metal itself is more likely to be corroded than the carbon material. In particular, it has been reported that the aluminum base material has a very high corrosion rate (RL Rorup and NE Vanderbo
rgh (LANL), Mater. Res. Soc. Symo. Proc., 393 (199
5) etc.). If metal ions generated by corrosion enter the electrolyte membrane, the ionic conductivity of the membrane may be reduced and the performance of the battery may be affected.

Japanese Unexamined Patent Publication (Kokai) No. 11-162478 discloses a method of improving corrosion resistance by plating a noble metal on the entire surface of a metal separator. This method has no problem with respect to the separator performance, but increasing the thickness of the anticorrosion coating causes high cost and is not practical. It is necessary to thin the precious metal plating layer to reduce the cost, but if the layer thickness is thinned during wet plating, fine pinholes will be generated and cause corrosion, and it will be produced by dry plating (evaporation, sputtering, etc.). The efficiency is poor and the uniformity of the coating is also poor.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a metal fuel cell separator which is excellent in corrosion resistance and electrical conductivity and can be manufactured at low cost.

[0009]

A fuel cell separator of the present invention is made of a metal base material and has a contact surface with an electrode or a current collector and a gas ventilation groove, and Ag, Ti, Ni is provided on the contact surface. Or C
r, or a first layer film made of an alloy containing it is formed, and a second layer film made of Au, Pt or Hg or an alloy containing it is further formed on the first layer film. Is characterized by.

From the viewpoints of cost and chemical stability, as a material used for the anticorrosive film of the separator, among the precious metals,
Ag is particularly suitable. However, high electrode potential in the fuel cell (potential effect of electrode and overvoltage effect when current flows), high temperature (around 80 ° C), high humidity (humidity 100%), pH change due to H ⁺ migration (pH 4 or less) ) In a harsh environment such as Ag
Are easily oxidized to form oxides such as Ag ₂ O and Ag ₂ O ₂ . The electric resistance of this oxide is very high, and when the oxide is formed, the collector's current collecting function is lost due to an increase in the surface electric resistance of the anticorrosion coating. Therefore, until now, Ag coatings have not been applied to separators. In the present invention, the first layer film made of Ag or the like is used as a film that blocks corrosion (does not proceed continuously). Au, Pt or
A second layer film made of Hg or an alloy containing it is formed. The second layer film plays a role of reducing electric resistance.
That is, if the second-layer coating is in a perfect state with no pinholes or breaks, it is useful for corrosion protection, but if it is broken, it serves as an electrical contact point to reduce the electrical resistance. When the second layer coating is damaged, the oxides such as Ag ₂ O and Ag ₂ O ₂ block the corrosion, so even if the thickness of the coating made of Au is significantly reduced compared to the conventional one, corrosion resistance It is possible to obtain an excellent fuel cell separator. In the present invention, the film thickness of the second layer coating is 0.01
It is preferably about 1.0 μm.

In the fuel cell separator of the present invention, it is preferable that an intermediate film made of a metal other than Ag or an alloy containing the same is formed between the metal base material and the first layer film. Intermediate coating is Cu, Ni, Ti, Cr, Fe, Zn and Co
It is preferably composed of a metal selected from the group consisting of or an alloy containing the same. From the viewpoint of corrosion resistance, the first layer coating is
It is preferable to contain 0.05 to 5.0% by mass of Pt. Also,
In order to suppress the migration described below, it is preferable that the first layer coating contains 0.05 to 5.0 mass% Pd. The first layer coating is particularly preferably 0.05 to 5.0% by mass of Pt.
And 0.05 to 5.0 mass% Pd.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION The fuel cell separator of the present invention is made of a metal base material and has a contact surface with an electrode or a current collector and a gas ventilation groove. At least on the contact surface of the separator
A first layer film made of Ag, Ti, Ni or Cr or an alloy containing Ag, Ti, Ni or Cr is formed. The first layer coating may be formed on a portion other than the contact surface of the separator. It is not necessary to form the first layer film on the entire surface of the separator, but it is preferable to form it on the entire surface particularly when the metal base material is a material such as Al that is easily corroded. Further Au, Pt or H on the first layer coating
A second layer film made of an alloy containing g or Au, Pt or Hg is formed. The second layer coating may be formed at least on the contact surface, and when the first layer coating is formed on a portion other than the contact surface, it is not necessary to form it on the entire surface of the first layer coating. For example, the first layer film may be formed on the entire surface of the separator and the second layer film may be formed only on the contact surface. INDUSTRIAL APPLICABILITY The separator of the present invention can be used for various fuel cells, and is particularly suitable for on-vehicle fuel cells for powering automobiles.

FIG. 1 is a partial schematic view showing an example of a fuel cell including the fuel cell separator of the present invention, and FIG. 2 is shown in FIG.
It is the partial schematic diagram which expanded the part shown by inside "A". Figure 1
The fuel cells shown in 1 and 2 are configured by stacking unit cells 1 each including a solid electrolyte 2 and an anode 3 and a cathode 4 provided on both sides of the solid electrolyte 2 with a separator 5 interposed therebetween. Normal,
Both ends of the stack are connected to an external circuit (not shown). A current collector may be installed between the electrode and the separator. Gas vent grooves 8 and 9 are processed in the separator 5. Normally, fuel gas is supplied to the passage formed by the gas ventilation groove 9 and the anode 3, and oxidant gas is supplied to the passage formed by the gas ventilation groove 8 and the cathode 4.

A first layer coating 6 is formed on the surface of the separator 5 shown in FIGS. 1 and 2, and a second layer coating 7 is further formed thereon. The contact surface of the separator 5 with the electrode or the current collector is required to have corrosion resistance and electrical conductivity, and the other surfaces are required to have only corrosion resistance.

The shape of the contact surface may be any shape suitable for contacting the electrodes of the fuel cell or the carbon paper, carbon cloth, etc. of the primary current collector, and is not limited by the drawings. Gas vent groove is machined, pressed, precision cast,
It may be formed into a predetermined pattern by a method such as chemical polishing (etching) and electrolytic polishing. Although the shape of the gas ventilation groove is U-shaped in the figure, it is not particularly limited as long as the reaction gas passage can be formed in the portion in contact with the electrode, and the reaction gas ventilation resistance is small and the power generation efficiency is high. It is preferable to set as follows. Normally, the depth of each gas vent groove is 0.2-2 mm
The width is preferably 0.5 to 5 mm. Hereinafter, the metal base material, the first layer coating, the second layer coating, and the intermediate coating used in the present invention will be described in detail.

(A) Metal Base Material As the metal base material, a general metal plate such as a carbon steel plate or a SUS steel plate may be used. When the separator is applied to an on-vehicle fuel cell of an automobile, it is preferable to use a metal plate made of a metal such as aluminum, titanium, magnesium, etc., which is lightweight and has high specific strength, or an alloy thereof as a metal base material. Above all,
A metal plate made of aluminum or an aluminum alloy is particularly preferable. To prevent reaction gas crossover (a phenomenon in which fuel gas permeates the electrolyte and leaks to the air electrode),
It is preferable to use a metal plate without defects such as through holes.
Although the thickness of the metal base material is not particularly limited, it is preferably 0.5 to 3 mm when used in a vehicle fuel cell.

(B) First layer film The fuel cell separator of the present invention has Ag, T on the contact surface.
From i, Ni or Cr, or alloys containing Ag, Ti, Ni or Cr
Has a first layer coating. Ag, Ti, Ni, Cr and their alloys
Has excellent corrosion resistance, but it may be affected by voltage application, oxidizer, etc.
When oxidized ₂O, Ag₂O₂Oxides such as are generated on the surface
The contact electric resistance of is greatly increased. At this time,
The corrosion rate of the film is further reduced and the progress of corrosion is suppressed.
It That is, the first layer coating is Ag, Ti, Ni or Cr, or its
It may include an oxide of the alloy. The thickness of the first layer is
0.5 to 50 μm is preferable, and 2 to 20 μm is preferable.
More preferable. The first layer film has no defects or pinholes.
It is desirable to be precise.

Among Ag, Ti, Ni, Cr and alloys thereof, Ag and alloys containing Ag are particularly preferable. Examples of alloys containing Ag include Ag-Pt alloys, Ag-Au alloys, Ag-Pd alloys, and the like. The mass ratio of Ag, Ti, Ni or Cr in these alloys is preferably 99.0 to 99.9 mass%.

The first layer coating is electroplating, electroless plating,
It can be formed by a method such as vapor deposition and sputtering, and is preferably formed by electroplating or electroless plating.
It is known that the current efficiency when depositing Ag is higher than that of other metals. That is, the gas generation reaction is suppressed. Therefore, when Ag is plated, a dense Ag first layer film can be obtained by selecting a plating bath and plating conditions with high current efficiency.

By adding Pt, Ni, Cu, Co, W, Au or the like to the first layer film, the oxidation resistance and hardness of the first layer film are improved and more excellent fuel cell characteristics can be obtained. Above all, 0.
It is preferable to add 05 to 5.0% by mass of Pt to the first layer film. In particular, when the Ag-Pt alloy film is formed, the corrosion resistance is significantly improved as compared with the Ag single film.

The fuel cell separator is usually exposed to water,
And it is placed in an environment with an electric field. In Ag in an aqueous solution, a phenomenon of migration of metal atoms may occur especially in the presence of a voltage. Due to the movement of the metal atoms, Ag concentrates on the active sites and recrystallizes and grows. As a result, the film may become non-uniform and the corrosion resistance of the separator may decrease. This migration can be suppressed by adding Pd, Sn, Zn or the like to the first layer film to increase the number of active sites. Above all, it is preferable to add 0.05 to 5.0 mass% of Pd to the first layer film. In particular,
When the Ag-Pd alloy film is formed, it shows excellent fuel cell characteristics.

The first layer coating contains 0.05 to 5.0 mass% Pt and 0.05.
By adding Pd in an amount of up to 5.0% by mass, it is possible to simultaneously improve the corrosion resistance of the first layer coating and suppress migration. When the Ag-Pt-Pd alloy film is formed, the fuel cell using this separator exhibits particularly excellent fuel cell characteristics.

(C) Second Layer Coating The fuel cell separator of the present invention has the above A coating on the first layer coating.
It has a second layer coating composed of u, Pt or Hg or an alloy containing Au, Pt or Hg. As mentioned above, the increase of surface contact resistance due to the oxidation of the coating such as Ag affects the current collection effect, but the contact electrical resistance should be kept low by forming the second layer coating on the first layer coating. You can That is, the second layer film is formed for the purpose of maintaining the contact electric resistance value at a low value rather than for corrosion protection. Therefore, the film thickness of the second layer film may be thin and does not need to be a dense film. By forming a dense second layer film, more excellent corrosion resistance can be obtained while maintaining a low contact electric resistance value. Usually directly on the metal matrix
Adhesion is improved by forming it through the first layer film rather than forming a layer made of Au or the like.

The second layer coating may be formed on the entire surface of the metal base material or only on the contact surface. For example, the first layer film may be formed on the entire surface of the separator and the second layer film may be formed only on the contact surface. The thickness of the second layer film is 0.01-1.0 μm
Is preferred. If the film thickness is less than 0.01 μm, the effect of maintaining the contact electric resistance at a low value is not sufficient, and if it exceeds 1.0 μm, cost increase cannot be avoided.

The second layer coating is preferably Au or Pt, or
It is made of an alloy containing Au or Pt. Even if Pt or an alloy containing Pt is used, the same level of contact electric resistance and corrosion resistance as when using Au or an alloy containing Au is obtained, but from the viewpoint of cost, it is preferable to use an alloy containing Au or Au. Particularly preferred.

The second layer film is electroplated, electroless plated,
It can be formed by a method such as vapor deposition and sputtering, and is preferably formed by electroplating or electroless plating.

(D) Intermediate film Generally, when different metal materials or conductive materials are brought into contact with each other, an electron barrier is generated by the surface energy, so that a contact electromotive force may be generated at the material interface.
This electromotive force may make it difficult to form a film on the surface, reduce the bond strength of the film, or cause corrosion between layers. Therefore, in the present invention, it is preferable to form an intermediate coating made of a metal other than Ag or an alloy containing the same between the metal base material and the first layer coating. By forming the intermediate coating, the bonding strength, compactness and corrosion resistance of the first coating can be improved.

From the viewpoints of bonding strength, denseness and corrosion resistance, the intermediate film is preferably made of a metal selected from the group consisting of Cu, Ni, Ti, Cr, Fe, Zn and Co or an alloy containing the same. In particular, when an Ag film is formed on the surface of an Al substrate, an excellent effect can be obtained by forming such an intermediate film.

The thickness of the intermediate coating is preferably 0.1-10 μm. If the film thickness is less than 0.1 μm, it is difficult to form a dense film, and if it exceeds 10 μm, the cost increases. The intermediate film can be formed by a method such as electroplating, electroless plating, vapor deposition, and sputtering.

[0030]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1 After etching and cleaning the surface of an Al metal plate having a thickness of 5 mm, zinc substitution treatment, Ni electroplating, Ag electroplating and Au were performed.
Electroplating was sequentially performed to form a Ni film, an Ag film, and an Au film, to manufacture the separator of Example 1. The Ni film thickness is 5 μm, the Ag film thickness is 3 μm, and the Au film thickness is
It was 0.4 μm.

Example 2 The surface of an Al metal plate having a thickness of 5 mm was etched and washed, and then zinc substitution treatment, Ni electroplating, Ag-Pd alloy (Pd content: 1% by mass) electroplating and Au electroplating were performed. This was sequentially carried out to form a Ni film, an Ag-Pd alloy film and an Au film to produce the separator of Example 2. The Ni film thickness is 5
μm, Ag-Pd alloy film thickness is 3 μm, Au film thickness is 0.4
μm.

Comparative Example 1 The surface of an Al metal plate having a thickness of 5 mm was etched and washed, and then zinc substitution treatment, Ni electroplating and Au electroplating were sequentially performed to form a Ni film and an Au film, and Comparative Example 1 The separator of was produced. The Ni film had a thickness of 5 μm and the Au film had a thickness of 0.4 μm.

Power Generation Test Examples 1 and 2 and Comparative Example 1 obtained as described above
Assemble each separator into the fuel cell,
A power generation test was performed for 100 hours under conditions of ° C and 0.65V. FIG. 3 shows the current density of each fuel cell at the start of the power generation test and after 100 hours of power generation. As is clear from FIG. 3, Comparative Example 1
In comparison with the fuel cell using the separator of Example 1, the fuel cells using the separators of Examples 1 and 2 according to the present invention show a higher current density, and in particular, the decrease of the current density after 100 hours of power generation is significantly suppressed, and the excellent corrosion resistance is obtained. showed that.

[0035]

As described in detail above, the fuel cell separator of the present invention comprises Ag, Ti, Ni or Cr, or a first layer film made of an alloy containing it, and Au, Pt or Hg, or It has a second layer film made of an alloy containing. Since an oxide film such as Ag is generated and passivated at the damaged part or the pinhole part of the second layer film, the corrosion of the metal base material can be prevented. Therefore, according to the present invention, the film thickness of the film made of Au or the like can be made significantly thinner than the conventional one, and the cost can be greatly reduced. That is, the fuel cell separator of the present invention can be manufactured at low cost, has low contact resistance, and has excellent corrosion resistance and electrical conductivity.

[Brief description of drawings]

FIG. 1 is a partial schematic view showing an example of a fuel cell including a fuel cell separator of the present invention.

FIG. 2 is a partial schematic diagram in which a portion indicated by “A” in FIG. 1 is enlarged.

FIG. 3 is a graph showing a result of a power generation test performed in an example.

[Explanation of symbols]

1 ... single battery 2 ... Solid electrolyte 3 ... Anode 4 ... Cathode 5 ... Separator 6 ... First layer film 7: Second layer film 8, 9 ... Gas vent groove

Claims (7)

[Claims] 1. A fuel cell separator comprising a metal base material and having a contact surface with an electrode or a current collector and a gas vent groove, wherein Ag, Ti, Ni or Cr or an alloy containing the same is provided on the contact surface. A fuel cell characterized in that a first layer film made of is formed, and further a second layer film made of Au, Pt or Hg, or an alloy containing it is formed on the first layer film. Separator. 2. The fuel cell separator according to claim 1, wherein Ag is provided between the metal base material and the first layer film.
A fuel cell separator, wherein an intermediate film made of a metal other than the above or an alloy containing the same is formed. 3. The fuel cell separator according to claim 2, wherein the metal other than Ag is Cu, Ni, Ti, Cr, Fe, Zn.
And a separator for a fuel cell, which is selected from the group consisting of Co and Co. 4. The fuel cell separator according to claim 1, wherein the second layer film has a thickness of 0.01.
A separator for a fuel cell, which is characterized by having a thickness of ~ 1.0 μm. 5. The fuel cell separator according to claim 1, wherein the first layer coating is 0.05 to 5.0.
A fuel cell separator comprising Pt in an amount of mass%. 6. The fuel cell separator according to claim 1, wherein the first layer coating is 0.05 to 5.0.
A fuel cell separator comprising Pd in an amount of mass%. 7. The fuel cell separator according to claim 1, wherein the first layer coating is 0.05 to 5.0.
A fuel cell separator comprising Pt in an amount of 0.05% by mass and Pd in an amount of 0.05 to 5.0% by mass.